Because of their excellent adhesion and rigidity properties, epoxy resins have been used extensively as matrix resins for composite materials. In particular, a composition that is mainly composed of N,N,N',N'-tetraglycidyl diaminodiphenylmethane and 4,4'-diaminodiphenyl sulfone has been used most extensively since 1972 as a matrix resin for high-performance composite materials.
However, the elongation of this resin composition is so small that composite materials produced by thermally curing the prepreg made of it are incapable of stretching in conformity with carbon fibers, aramid fibers, glass fibers or other recently developed high-elongation reinforcing fibers having an elongation at break of 1.5% or more. In other words, these composite materials have a lower tensile elongation than the reinforcing fibers. Although they exhibit satisfactory compressive strength at about 82.degree. C. after moisture absorption, their compressive strength after impact is too low to justify their use as primary structural materials.
Materials having high compressive strength after impact have been reported in the 5th SAMPE European Chapter (1984) Paper 15 (T. Tattersall), but they are far from being suitable for practical use because, for one thing, they do not have satisfactory compressive strength after moisture absorption and, for another, the volume fraction of fibers cannot be increased on account of the need to insert interleaves or intermediate layers.
Under these circumstances, the present inventors conducted intensive studies in order to develop an epoxy resin composition which is used in prepreg for a composite material that would impart excellent compressive strength characteristics both at about 82.degree. C. after moisture absorption (.gtoreq.110 kg/mm.sup.2) and after impact (.gtoreq.27 kg/mm.sup.2). The present invention has been accomplished on the basis of these efforts.